In FIG. 1 of the appended drawing a simplified Global System for Mobile communications, a GSM system for short, is shown as a block diagram. The Network Subsystem, NSS, includes a mobile services switching centre MSC 2 which is connected to other mobile services switching centres, and directly or through a Gateway Mobile Services Switching Centre, a GMSC system interface the mobile network is connected to other networks, such as a Public Switched Telephone Network, PSTN 4, an Integrated Services Digital Network, ISDN 6, other Public Land Mobile Networks, PLMN 10, and packet switched public data networks, PSPDN 12, and circuit switched public data networks, CSPDN 14. In the mobile services switching centre MSC 2 there are Network Interworking Functions, IWF 16, for matching the GSM network with the other networks. Through an A-interface the NSS network subsystem is connected to a Base Station Subsystem, BSS, which includes base station controllers BSC 20, each one of which controls the base transceiver stations BTS 22 connected to them. The interface between the base station controller BSC and the base transceiver stations BTS connected to it is an A bis interface. The base transceiver stations BTS 22 for their part are connected over a radio path with mobile stations MS 24 across the radio interface. The operation of the whole system is monitored by an Operation and Maintenance Centre, OMC.
The mobile station MS 24 sends speech or user data across the radio interface on a radio channel at standard rates of e.g. 13 kbit's or 5.6 kbit/s. Speech coding is used in the speech transmission to achieve a lower transmission rate than typically in telephone networks, whereby the band width needed by the radio link on the radio path is reduced. The base transceiver station BTS 22 receives the data of the traffic channel and transmits it into the 64 kbit/s time slot of the PCM line. Into the same time slot, that is, channel, are also placed three other full-rate traffic channels of the same carrier wave, so the transmission rate per connection will be 16 kbit/s. For half-rate traffic channels the transmission rate is 8 kbit/s per connection. A transcoder/rate adapting unit TRAU 26 converts the coded 16 kbit/s or 8 kbit/s digital information to the 64 kbit/s channel, and on this channel the data is transmitted to an IWF 16 unit which is located in the mobile services switching centre MSC 2 and which performs the required modulation and rate conversion, whereupon the data is transmitted to some other network. Thus, the user data is transmitted over fixed connections in the uplink direction from base transceiver station BTS 22 to base station controller BSC 20 and to mobile services switching centre MSC 2 and, correspondingly, the data to be relayed to mobile station MS 24 is transmitted in the downlink direction from mobile services switching centre MSC 2 through base station controller BSC 20 to base transceiver station BTS 22 and from there further over the radio path to mobile station MS 24.
In the GSM system, a Channel Codec Unit, CCU, of the base transceiver station performs a conversion of the signal received on the radio channel to the PCM time slot channel of the trunk line running over the A bis interface and a conversion of the frame structure of the signal received over the A bis interface into a form which can be transmitted on the radio channel. The transcoder unit TRAU 26 performs the conversion operations on the signals to be transmitted across the A-interface. The transcoder/rate adapting unit TRAU 26 is often located far from the base transceiver station 22, e.g. in connection with the base station controller BSC 20.
In a digital mobile system, speech is generally coded into a digital form by using low rate speech coding. Nowadays the SM system uses Full Rate FR 30 coding at a transmission rate of 13 kbit/s, Half Rate HR 32 coding at a transmission rate of 5.6 kbit/s, Enhanced Full Rate EFR 30′ coding at a transmission rate of 12.2 kbit/s and Enhanced Half Rate EHR 32′ coding. The enhanced speech codings 30′, 32′ are so advanced that the quality of speech is not significantly reduced in them.
Thus, speech coding is performed in the mobile station and on the mobile network side in the transcoder unit TRAU 26. The speech information to be transmitted is one of the parameters of the speech coding method. In modern GSM systems the TRAU 26 transcoders are of several different types of coding, e.g. full rate, half rate or double acting, which is able to change from one rate to another. The transcoders convert the speech from a digital format into another, e.g. they convert 64 kbit/s A-law PCM arriving from the exchange over the A interface into 13 kbit/s full rate FR 30 coded speech for transmission to the base transceiver station line, and vice versa. In a call between two mobile stations PCM speech samples are transmitted from one transcoder to another, which codes them by a speech coding method which is used on the radio path. Repeated coding and decoding of the speech signal during the transmission will distort the speech signal, for which reason this coding-decoding chain, which is called tandem coding, is usually avoided.
The Finnish Patent Application FI-951807 presents transmission of speech frames, which have not been decoded, between transcoders and thus barring of tandem coding in the transcoder, when the call is one between two mobile stations, that is, a Mobile to Mobile Call, MMC. In the solution according to the application, the coded speech parameters are sent on the PCM time slot sub-channel without any decoding and coding in the TRAU transcoders of the mobile station network. Tandem coding is avoided by relaying with minor changes the frames coming from one base transceiver station BTS 22 through these tandem connected TRAU 26 transcoders to another base transceiver station BTS 22′. The receiving transcoder will perceive from these minor changes in the frame that coding need not be done, and it will relay further the received speech parameters of the frame as such.
The Finnish Patent Application FI-960590 presents a transmission adaptation for a connection between exchanges. In the solution according to the application, a speech signal is transmitted coded by a speech coding method on a sub-channel of a PCM channel. The speech coding for the connection between exchanges is chosen according to the speech coding of the TRAU frames of the A-interface, except if the speech coding of the TRAU frames received from the A-interface is different from the speech coding of TRAU frames received from other transmission equipment, that is, if the parties to the call are using different speech codings. FIGS. 2a–2c illustrate this adaptation of the transmission for a connection between exchanges in a few cases shown as examples. The speech coding used in each part of the transmission path is marked in the figures, in this example they are full rate FR 30 and half rate HR 32 speech coding. In FIG. 2a both mobile stations MS1 24 and MS2 24′ use the full rate FR 30 speech coding method. Hereby the speech parameters are transmitted through the mobile station network as unchanged full rate speech parameters. In FIG. 2b both mobile stations MS1 and MS2 use a half rate HR 32 speech coding method. Hereby the speech parameters are transmitted through the mobile station network as half rate speech parameters. In the case shown in FIG. 2c, mobile station MS1 uses half rate HR speech coding while mobile station MS2 uses full rate FR 30 speech coding. In this situation, a change is made at the mobile services switching centre MSC1 end to full rate speech coding and the necessary decoding and speech coding are performed.
A problem with the presented transmission situations is the need of transmission capacity, especially on the transmission connection between the base transceiver station and the network transcoder. The transmission in the mobile station network of speech parameters of the mobile station using full rate speech codec requires a full rate channel, which cannot be transmitted, if on the transmission connection e.g. only a half of that transmission capacity is available, which is required by a full rate signal.